In the field of optical communications, there has been practically used a technique of realizing an optical modulator by employing a substrate-type optical waveguide. For example, a substrate-type optical waveguide having a phase modulation function can be realized by forming, in a core of the substrate-type optical waveguide, a P-type semiconductor region and an N-type semiconductor region. In such a substrate-type optical waveguide, an effective refractive index of the core changes in accordance with a signal voltage applied across the P-type semiconductor region and the N-type semiconductor region. To apply a signal voltage, for example, a coplanar line is used which is provided on a surface of the substrate-type optical waveguide.
For example, Patent Literatures 1 through 3 each disclose a substrate-type optical waveguide having a phase modulation function.